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Post by endru on Aug 27, 2013 21:30:04 GMT -5
I just installed a new big valve head. Took it for a test run today and when I got back, I noticed smoke coming out from the engine compartment under the seat while it was idling. I discovered that it was coming from the breather tube on the valve head. It's a significant amount of smoke. Other symptoms which may be related are that it was not idling well during the ride, and I couldn't get any rpm response from adjusting the A/F mixture screw. I have 2 theories. The head may be bad and one of the valves is letting gasses blow by by not seating properly. But that to me wouldn't explain the amount of smoke coming out the hose. The other theory of course is that the rings blew somehow and exhaust is now going past the piston, into the crankcase, back up through the cam chain opening and out the tube. But why suddenly with a new head would the rings blow out? Maybe this is normal for a new head and it will fix itself after it gets broken in. What do you all think?  |
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Post by rae4180 on Aug 27, 2013 22:27:55 GMT -5
sometimes new heads have a factory coating on theam that needs burned off after being on the road for a while. you my be lucky and thats all it is. just take it easy and dont ride full wot for a while.
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Post by skuttadawg on Aug 28, 2013 1:04:24 GMT -5
Did you install a larger carb or at least a larger main jet in the carb ? If not you are running waaaaay too lean and will burn it up . Did you install the rings correctly ? Check the valve clearances . Did you use oil or assembly lube on the piston or install it dry ? When my old scoot fizzled on day two of owning it I took it back to see what was wrong with it . The dealer pulled off the hose from the valve cover and noted it smoked . It should not smoke . I had a hairline crack in the piston . Running too lean is one of the major causes of scooter engine failure since it runs hotter than it should since it is not getting enough fuel . This can cause the piston to crack , burn a hole through the top and swell up to where it seizes and ruins the piston , rings and cylinder . Pull out the sparkplug and compare its color with this chart www.4secondsflat.com/plug_chart.html If it is whiteish then you need to upjet . Here is a jet kit which will help you tune it and have extra jets in case you change the airfilter or exhaust and need to upjet again later .http://www.partsforscooters.com/114-49-QMB139-Main-Jet-Kit?sc=34&category=133078 Note the size of your idle jet as you may want to go up a size . My 2T never idled good for a long time and it got worse with a BBK . I had a 40IJ and went with a 45IJ and wow it was such an improvement . I think the 4Ts use a 32 or 35 IJ Check for leaks in the intake tubing and exhaust too |
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Post by endru on Aug 28, 2013 6:20:51 GMT -5
[replyingto=skuttadawg]skuttadawg[/replyingto]I only did the head replacement, the big bore piston and cylinder had been installed for a few months now and running well. When I pulled off the old head I didn't take the whole top end off, but inspected the cylinder wall and piston head and they looked good to the naked eye. Valve clearance I set at the typical .10mm intake and .12mm exhaust (that's .004 and .005 inches). In the end I did not upjet. With my stock 19mm carb I always seemed to get rich mixtures with the jet sizes people typically use for a bbk. So before the new head I was obliged to run with a mere 80. I had no backfiring and the plug looked good enough... not coffee colored but not white. So with the new head I jumped right up to an  , and the scooter ran like crap, stuttering at anything more than 1/4 throttle. So I went up to a 92 and it was much worse, then back down to the 80 since I don't have an  . I just now went out to do the plug chop and it's definitely lean. So the question is, is the smoke coming because I completely ruined the top end, or will that go away if I upjet to an ? I guess I'll have to just try it and find out. Would a compression test be a good diagnosis of the condition of the piston and rings? I'm just appalled that this kind of damage could happen after a 10 minute test ride around town. Maybe with a 70 main or something ridiculous like that. Thanks for the quick responses! |
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Post by chase50 on Aug 28, 2013 16:52:28 GMT -5
I had smoke coming from my vent hose on the first start after installing my BBK, but it went away after the second or third run. On a separate note, maybe you're not getting enough air for the # .... are you using stock airbox? |
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Post by endru on Aug 28, 2013 19:48:58 GMT -5
I had smoke coming from my vent hose on the first start after installing my BBK, but it went away after the second or third run. On a separate note, maybe you're not getting enough air for the # .... are you using stock airbox? Now that's good to hear, maybe my rings aren't shot after all. I should have put that in my signature too - I'm running a uni-style filter. The only thing I can figure is that the 19mm carb is really restricting the airflow - at least that's what was suggested to me in another thread a while back when I first installed my BBK and was shocked about the 80 main working best. Still, I'm not running that 80 any longer on this setup, I'll wait until I can at least get an in there. If all else fails I'll spring for a compression tester and see what that tells me. |
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Post by chase50 on Aug 28, 2013 21:56:17 GMT -5
I wouldn't think there'd be that much difference between 19 and 20mm carburetors. I trust you adjusted your AF mixture screw. Is your uni attached directly to your carb? No extensions or mods of any kind between the uni and the carb?
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Post by endru on Aug 29, 2013 20:47:53 GMT -5
[replyingto=chase50]chase50[/replyingto]I've always wondered what kind of a difference a 20mm carb makes over a 19mm carb. You're right, it doesn't seem like much. But then again, a 20mm usually takes a 40mm air filter (up from 38mm) so that might be the bigger difference than the 20mm business end of the carb. Anyway, yeah my filter is right on the carb, clamped down straight and tight. I sprayed around the intake and exhaust with carb cleaner while it was running to check for leaks, but I couldn't hear anything out of the ordinary. So, apparently I did have an jet in my cache. I popped that bad boy in thinking all my issues would be gone. Wrong. It splatted and stuttered on my test ride just as bad as the bigger jets. BUT the breather tube didn't seem to be smoking when I got back. I really hope it's like chase50 said and the smoking will just go away on its own. It still has a horrible idle, in fact I stalled out at one point and could not get the scoot to start up again until I had turned the A/F screw almost all the way out. Even then it doesn't idle well. My plan is to buy a 38 idle jet (which is a different version than the typical Keihin idle jets), clean the carb, try running with the stock exhaust and/or without an air filter and see if this tells me anything. A compression tester from harbor freight isn't that expensive but they have super crappy reviews, and my local Advance Auto doesn't stock them as loaners. BTW what's a typical compression reading for a 50mm bbk and head? |
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Post by endru on Aug 31, 2013 16:18:33 GMT -5
Right now I just want to throw my scooter over a cliff. I just simply cannot get it to run right. Nothing I tried helped. Among many other things, the carb is cleaned and the stock exhaust is on, but it still has an ugly idle that I can't fix with any carb adjustments, and the scoot sputters and skips when I give it throttle.
I'm beginning to wonder if the idling problem and the poor performance when giving it throttle are caused by the same thing. Could it be that my carb is crap? Or is the intake on it too small for my current setup? Could my cdi be bad and misfiring at all rpm ranges? I guess the only way to know is to buy a new carb and/or a new cdi, but if neither solves the problem then I just spent all that money for nothing.
So I guess my questions for the forum are 1) Can a bad cdi cause rough idle and what sounds like sputtering/misfiring, or does a bad cdi just stop firing altogether? 2) Is a 19mm carb ill-suited to a 50mm bbk, large valve head, and free flowing intake/exhaust setup, to the point that it would cause the described problems?
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Post by buford1488 on Aug 31, 2013 17:15:03 GMT -5
were did you get the big-valve head? the big valve heads i do need a 24mm carb...
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Post by endru on Aug 31, 2013 20:16:03 GMT -5
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Post by skuttadawg on Aug 31, 2013 20:57:29 GMT -5
The big valve head at Scrappys www.scrappydogscooters.com/139QMB_50cc_Performance.html says a 24mm carb will work with it . On my 2T it always had idle bugs like having to adjust the idle speed and it got worse with a BBK . I went from a 40IJ to a 45 and it purrs now . So perhaps a larger IJ will help you as well . If you get a Mikuni , Keihin or other brand name carb they often work better than generic ones . If you have the cash there is the option of EFI like here www.ecotrons.com . My Echarm has Ducatii EFI and WOW it is soooo much better than a carb in every aspect ! |
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Post by buford1488 on Sept 1, 2013 8:16:23 GMT -5
i hate it when these cheap e-bay places call it a RACING HEAD when they do nothing to the ports!! any-way what is the measurement of the intake port? if it's 19mm use a 19mm carb and play with the jet or clip.BUT first see what the intake port measures.. and get rid of the spacer/insulater. btw alot of heads that come from mederoca place's the valves LEAK.
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Post by sickopsycho on Sept 1, 2013 16:00:58 GMT -5
[replyingto=awhitlock]endru[/replyingto]While I don't have much to offer as far as advice in your problem- I can vouch for the Harbor Freight compression tester- While I wouldn't rely on it for everyday use at a performance car garage, the one I purchased about 2 years ago is still giving me reliable service. I have only used it maybe a dozen times or so but when I checked it against a Snap-On compression tester about a year ago it was pretty much spot on. Certainly gives me a good idea of what's going on in my scoot. Just remember- when you crank the engine while testing you need to have the throttle wide open- you will get a false reading otherwise. Anything around 150-200 is in good shape, 120-150 you got a bit of a problem and below 120 start looking to fix something and if it runs under about  -100 I'd be very surprised. =) |
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Post by endru on Sept 1, 2013 19:36:35 GMT -5
skuttadawg - a 45 idle jet? Wow that's big, but I'm guessing the 2T's run generally larger idle jets than 4T's. I ordered a 38 idle a while back, but discovered it was the wrong fit. Apparently Keihin carb jets are not all made alike. So I have a 38 ready to go for the new carb. EFI - you probably remember a discussion about that from the old scootdawg site. It intrigued me then, but I'd probably upgrade to a motorcycle before I made that kind of investment on my scooter. @buford - Yeah "racing head" is hardly accurate. The only people I know of online that port heads is scrappys, and from what I've heard they do a great job. At some point I may see if I can send in this one to be ported. Might be fun to try out myself though... I have a dremel and I could take a practice run on the stock head first. Anyway, as far as intake port measurement, you bring up an interesting point. I will measure it then to get an exact number, but I can tell you now the spacer from the stock head matches the new port diameter, so it probably is only 19mm. And if I got a 24mm carb, it seems like it would bottleneck right there at the 19mm intake opening. I'll try it without the spacer once just to see what that does. Do you know specifically why that spacer might cause problems? I've got a gasket underneath it and there's the o-ring on the manifold, so if it's tightened down securely I can't imagine any outside air leaking in. sickopsycho - Thanks for the plug on the compression tester and the psi numbers, that's exactly what I needed. That's a lot of good info, thanks everyone. Does anyone now about the main jets question - are the 19mm carb main jets compatible with the 24mm carb?
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, and the scooter ran like crap, stuttering at anything more than 1/4 throttle. So I went up to a 92 and it was much worse, then back down to the 80 since I don't have an 
. I just now went out to do the plug chop and it's definitely lean. 
-100 I'd be very surprised. =)